Endogenous opioids are found throughout the body and are involved in a variety of homeostatic functions and movement control. Receptors that are regulated by endogenous opioids include delta (δ) receptors, kappa (κ) receptors and (μ) receptors, all of which are located in the brain and the peripheral nervous system and play a role in analgesia. Of these receptors, the (μ) receptors are located in the human gastrointestinal tract on myenteric and submucosal neurons and on immune cells of the lamina propria and play a role in gastrointestinal function.
Exogenous opioids, such as morphine, oxycodone, hydrocodone, buprenorphine and fentanyl, are commonly prescribed to treat both acute and chronic pain, as their action on the opioid receptors can provide effective analgesia. However, with respect to the (μ) receptors, the stimulating effect exogenous opioids have on these receptors may also cause an adverse pharmacodynamic response including bowel dysfunction that can be manifested by, e.g., decreased gastric motility, delayed gastric emptying, constipation, bloating and cramping. Other adverse pharmacodynamic responses associated with opioid therapy include nausea, vomiting, somnolence, dizziness, respiratory depression, headache, dry mouth, sedation, sweats, asthenia, hypotension, dysphoria, delirium, miosis, pruritis, urticaria, urinary retention, hyperalgesia, allodynia, physical dependence and tolerance.
Opioid-induced adverse pharmacodynamic responses in patients receiving opioid therapy for pain management can be particularly troublesome, as these patients are already trying to manage severe pain, and the added discomfort of adverse side effects can add to their distress. In some cases, the side effects may be so extreme that the patient would rather discontinue use of the opioid than continue to suffer with such side effects.
In the case of opioid-induced bowel dysfunction, current treatments include administration of laxatives, opioid antagonists and prokinetic agents. However, all of these treatments are not without risk. Laxatives, such as bisacodyl and psyllium, have a long history of safety and efficacy issues, and can themselves produce severe side effects such as dehydration and bowel obstruction Opioid antagonists, such as naloxone and naltrexone, while acting to suppress the receptors causing the bowel dysfunction, can reverse the desired analgesic effect of the opioid. Prokinetic agents, such as metoclopramide, may improve gastrointestinal motility but are associated with extrapyramidal effects, such as acute dystonic reactions, pseudoparkinsonism or akathisia.
There remains a need in the art for a composition and method to prevent or treat an opioid-induced adverse pharmacodynamic response that minimizes the issues of the current treatment protocols
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